The present invention relates to a process for the preparation of fairly high density polyolefin foams which predominantly have taut, closed cells and do not exhibit undesirable after-shrinkage.
It is known that polyolefin foams can be prepared by letting down a hot, pressurized homogeneous mixture which consists of a polyolefin and a volatile blowing agent. As a rule, the procedure is carried out in a continuous mixing apparatus with ejection of the polyolefin/blowing agent gel. When the mixture is let down into the atmosphere, the blowing agent evaporates with formation of a substantially closed-cell foam. In this process, as described in, for example, DE-B No. 25 24 196, it is only possible to obtain foams having a density of less than 30 g/l, which subsequently exhibit considerable shrinkage. When an attempt is made to increase the foam density by reducing the content of blowing agent in the gel, the heat of vaporization of the blowing agent when the mixture is let down is not sufficient to cool the polyolefin to such an extent that it solidifies. The foam therefore collapses.